Applications or devices using integrated circuits often provide a low power or standby mode of operation in which power usage is conserved. Integrated circuit designers may also strive to provide a power efficient integrated circuit. To provide such power savings, a portion of an integrated circuit not currently being used may be switched off.
To implement this switch off of a portion of a circuit, a power island is created. A power island may be a portion of circuitry that can be switched off by removing power to that portion. Typically, a power island comprises circuitry carrying out or associated with a particular function of an integrated circuit. The circuitry of the power island may be surrounded by switches to remove the power, as well as isolation cells to clamp floating inputs (and thus prevent leakage and uncontrolled logic values) of cells fed by the power island outputs.
The size or footprint of circuitry is an important consideration in some applications of integrated circuits. In integrated circuits with power saving capabilities, these switches and isolation cells make a significant contribution to the overall footprint of the integrated circuit.